Devices for converting radiant energy, such as optical energy, into electric energy presently generally take two forms, viz.: (1) semiconductors relying upon a barrier layer mechanism, and (2) pyroelectric devices wherein a ferroelectric is cyclically heated and cooled to provide corresponding changes in the capacitance and resistance of a capacitor including the ferroelectric.
Typically, the barrier layer semiconductor devices have relatively heavy doped semiconductor layers with energy gaps that, in essence, absorb certain wavelengths of interest and convert the energy of the absorbed wavelengths into electric energy. These devices are utilized as radiant, optical energy detectors for specific wavelengths of interest, as well as power generating solar cells. The major disadvantage of the semiconductor devices as radiant energy detectors is that the semiconductor element must usually be maintained at cryogenic temperatures to function effectively. It is frequently difficult to maintain a semiconductor device at a cryogenic temperature, whereby the usefulness of semiconductor radiant energy detectors is often limited. The major disadvantage of the semiconductor devices as solar energy converters is that such devices are relatively inefficient in converting the solar energy into electrical energy. The typical, actual maximum efficiency of such converters is generally on the order of 10%.
Pyroelectric devices are generally characterized by a ferroelectric dielectric that is positioned between a pair of electrodes to form a capacitor responsive to the optical energy. Typically, the ferroelectric material is periodically heated and cooled to cause a periodic variation in the capacitance and resistance of the capacitor. Since the ferroelectric materials have dipole layers extending completely through the dielectric, i.e., from one electrode to the other electrode, the dielectrics are strongly piezoelectric, making them sensitive to vibrations. Thereby, the pyroelectric detectors have a tendency to be noisy and frequently have relatively low signal to noise ratio outputs. In addition, the pyroelectric detectors often have detectivities below the level of the radiation impinging on the dielectric, thereby limiting their application in many systems.
In my copending application entitled "Apparatus For Converting Radiant Energy Into Electric Energy", Ser. No. 631,689, filed Nov. 13, 1975, there is disclosed an improved radiant energy detector and solar energy converter having higher detectivity and sensitivity than prior art detectors and greater efficiency than prior art energy converters. In the device disclosed in my copending application, a capacitor includes an ionic dielectric having a dipole layer only on or near the dielectric surface. The dielectric is generally selected from the group consisting of the rare earth trifluorides and trichlorides, and is preferably lanthanum trifluoride. A possible problem with the use of this class of materials is that it has relatively low breakdown voltages, on the order of 5 to 10 volts, regardless of the thickness of the dielectric layer; the breakdown voltage is dependent exclusively upon the dielectric material. A further possible disadvantage of the device disclosed in the copending application may be the difficulty of obtaining the specified rare earth trifluorides and trichlorides.
In Ser. No. 710,296, there is disclosed a device for converting radiant energy into electric energy which includes a capacitor responsive to the radiant energy, wherein the capacitor comprises a layer of an intrinsic or lightly doped semiconductor and at least one and preferably two insulating layers on the semiconductor layer. If the semiconductor is lightly doped, to increase the breakdown voltage of the capacitor, the doping is light enough that the semiconductor layer still functions effectively as an intrinsic layer. Intrinsic semiconductor layers, e.g., of germanium or silicon, with deep doped donors or acceptors, to a concentration of approximately 10.sup.17 to 10.sup.18 per cm.sup.3, or with shallow doped donors to a concentration of approximately 10.sup.13 to 10.sup.14 per cm.sup.3 are considered to be effectively intrinsic semiconductor layers. (Shallow donors or acceptors, by definition, are impurities having ionization energies small enough so they are completely ionized at room temperature. Thus for them there is a one-to-one correspondence between the impurity concentration and the carrier concentration. However, deep donors and acceptors, by definition, are not completely ionized at room temperature, so for them there is not a one-to-one correspondence with the carrier concentration.) The device is provided with a pair of contacts that are preferably formed as metallic contacts on the first and second insulating layers. There are no junction-type barrier layers in the semiconductor layer or between the insulating layers and the metal contacts; the only barrier layers reside at the interfaces between the semiconductor and the insulating layers. Such a configuration has an equivalent circuit including a pair of series capacitors having values commensurate with the capacitance of each of the insulating layers, a series resistor having a value equal to the semiconductor resistance, and a further pair of series capacitors having values equal to the capacitance of a pair of depletion and accumulation layers of the intrinsic semiconductor. In a case (Case I), corresponding to a low charging voltage, for a specified ambient temperature, three cases can be distinguished as a function of charging voltage applied between the contacts. The depletion and accumulation layers have effective thicknesses equal to the Debye shielding length of the semiconductor at each of its intersections with the insulating layers. In a second case (Case II), the voltage is large enough to form a depletion layer having a thickness exceeding the Debye shielding length and substantially smaller than the semiconductor thickness. For the third case (Case III), the voltage is so large that the depletion layer occupies practically the whole semiconductor thickness. The preferred operating conditions correspond to Cases I or II. The device will operate if they are biased into inversion.
Preferably, the device of the prior art, as disclosed in Ser. No. 710,296, now U.S. Pat. No. 4,058,729, utilizes any of the well known, and generally available, intrinsic semiconductors, such as silicon, germanium, gallium arsenide, diamond, silicon carbide and gallium phosphide; also, the insulating layers are preferably formed as oxides of the intrinsic semiconductor. The intrinsic semiconductors can be charged to relatively large voltages, on the order of 30 volts, without breaking down while still operating in Cases I and II defined earlier. Thereby, the devices of Ser. No. 710,296 can be initially charged to a higher voltage level than the devices described in my copending application, Ser. No. 631,689, thereby possibly providing greater detectivity with the device of the present invention than the device disclosed in the prior patent application. By utilizing lightly doped semiconductors, as described supra, the initial charging voltage can be increased further without causing breakdown of the semiconductor layer.
The resistance and capacitance of the intrinsic or lightly doped semiconductors vary in an exponential manner for Cases I and II in accordance with: EQU C=C.sub.o e.sup.-.spsp.E.sup.c/kT ( 1) EQU r=r.sub.o e.spsp.E.sup.r/kT ( 2)
where:
C.sub.o =static capacitance of the capacitor at high temperature, i.e., kT&gt;&gt;E, PA1 R.sub.o =the static, series resistance of the capacitance at high temperature, i.e., kT&gt;&gt;E, PA1 E.sub.c =the activation energy for the temperature dependent capacitance of the capacitor, PA1 E.sub.r =the activation energy of the temperature dependent resistance of the capacitor, PA1 k=Boltzmann's constant, and PA1 T=temperature in degrees Kelvin. Hence, by cyclically increasing and decreasing the temperature of the dielectric, by chopping radiation from a source of interest or solar radiation, the capacitance of the dielectric respectively increases and decreases, while the resistance of the dielectric respectively decreases and increases.